This invention relates to thread tension control devices for overedging sewing machines. More particularly, this invention relates to thread tension control devices wherein the needle threads are controlled to be at a relatively high tension and the looper threads to be at a relatively low tension during an overedging process and the needle threads are controlled to be at a relatively low tension and the looper threads to be at a relatively high tension during a process of forming thread chains which extend from the fabric material after its sewing, and wherein thread tension conditioning for each process can be regulated.
In the prior art is Japanese utility model publication 57-59105. According to this publication, two thread tension controllers are provided for both the needle thread passageway and the looper thread passageway (for two needle overedge sewing machines). Additionally, two more auxiliary thread tension controllers are provided for both the needle thread passageway and the looper thread passageway and these auxiliary tension controllers are actuated by a solenoid energized by signals from sensors which detect the leading edge and the trailing edge of the fabric material. Thus, proper thread tension for the overedging process and the thread chain forming process are regulated.
The aforementioned conventional overedging sewing machine provides two thread tension controllers for both the needle thread passageway and the looper thread passageway and two additional auxiliary thread tension controllers for both passageways. Thus, the mechanism is complicated and the controllers occupy substantial space and as a result the machine design is neglected.